The main reasons you and your car will get poor mileage is because of the following:
1. Driving faster than the speed limit. The drag co-efficient of the larger passenger car or truck of today still isn't fantastic. Therefore it requires more horsepower (and fuel) to displace air as you drive down the highway. It has been said that the difference between 55 MPH and 70 MPH can be 20% of your fuel costs. Frontal area is a very key factor in mileage and performance--just ask your NASCAR crew chief... yes, you will get better mileage if you follow that semi-trailer too closely, but that's not exactly a smart thing to do.
2. Tire inflation pressures incorrect. Although the manufacturer often recommends inflation pressures of a lower value, it requires more horsepower to roll a lower inflated tire down the road. You'll find that increasing inflation pressures so you get a more even tread wear pattern on your tire will also indicate the best 'tire contact' to the ground. Tires that wear more on the inside AND outside of the tread pattern indicate under inflation. A wear pattern in the centre of the tire indicates over inflation--but the mileage gains may offset tire replacement cost.
You want good tire contact across the tread, without increasing TOTAL tire contact patch to the ground. The greater the contact patch--the more friction--the more heat--the more total energy lost.
3. Engine state of tune. If you can't remember when you replaced your spark plugs or air filter--you may want to give them your attention. Stay away from 'fancy' plugs...all a spark plug does is provide a gap for the spark. Improper "spark plug reach" can inhibit mileage as much as 30%...so make sure the spark plug you buy was intended for the specific application. If there are any doubts--consult an application guide for your specific engine. The 8th digit in your VIN number identifies the engine family. A plugged air filter hurts performance...and if your vehicle has a MAF sensor, when you make it flow better--it'll actually use more fuel. So be conscious of your choices with FIPK retrofit kits or other inlet modifications.
4. Wheel Alignment. An alignment problem can be the equivalent of dragging your car sideways down the highway a couple of feet for every couple miles of driving. A real good indicator of alignment problems are the front tires. Here's a quick and simple test. Get a tire crayon and 'color' a 1" wide strip across the front of your tire from inside to outside. Do the same on the other front tire. Drive in a straight line for 100 feet (pavement or concrete) without turning your steering wheel . If the crayon is already scrubbed off the tire--you may want to consider a wheel alignment. This little test in no way indicates a proper alignment--but would indicate improper 'toe' alignment, which is the #1 tire wearing angle. This Toe check indicates if your tires are ( // --- \\ toe in) or ( \\ --- // toe out) in relation to each other. Because of the complexity of today's automotive suspensions--take the vehicle to the experts. Do not attempt to adjust anything yourself. Insist on a 4-wheel alignment for a FWD car -- and a thrust align, or two wheel alignment for a rear wheel drive solid rear axle vehicle.
5. Cold driving -- Warm up. There is no 'real' advantage to fully warming up your engine before driving, other than using more fuel. This also applies to extended idling, etc...it's all wasted fuel. Try to consolidate trips to the store, school, work, errands....careful planning and execution of a route can safe fuel (and a lot of unnecessary mileage). Driving like you stole the vehicle will never help mileage--drive your vehicle like you've got an egg between your foot and the gas pedal...accelerate slowly.
6. Incorrect tire size. When you install a larger tire than what was original to the vehicle, you are actually requiring the engine to develop even MORE torque to maintain rolling resistance down the highway. Although we can correct the speedometer and odometer--don't expect mileage gains.
7. Decrease AIR !! This flies in the face of what is sold out there today, but if you can decrease the amount of air entering the engine (due to the fact that this is a closed loop system as of 1996 OBD2) you will use less fuel, too. Ideally, you want to maintain the same horsepower required to combat air friction, rolling mass losses, etc...but do it with LESS AIR !!
Let me illustrate. A closed loop system maintains the air fuel ratio at 14.7 lbs air to 1 lb of fuel. This is maintained for idle, cruise and in some cases, even at wide open throttle.
If you have 60 grams/sec air entering the engine at cruise, you're using 4.08 grams of fuel per second. If the cylinder pressures are increased without creating harmful exhaust byproducts, or if these byproducts are controlled by the catalytic converter, we can gain mileage by decreasing airflow through the engine while enhancing the spark curve to maintain the same rear wheel HP.
On the chassis dyno, we can get vehicles down to 45-50 grams air/sec maintaining the same air fuel ratio, and same wheel HP as stock--and the overall result is less fuel consumed ! 45 g/secs works out to 3.06 g/fuel used in the same time period--or an almost 25% saving. We can't gain this in all vehicles of course--this example was resultant data from a 2007 6.0L HD 3500 GM truck.
This flies in the face of every aftermarket product out there claiming that it saves fuel--you will never gain mileage by increasing airflow through a gasoline engine in a mass airflow type system.
Because we carefully remap the spark curve with a remapped ECU, we in effect are increasing cylinder pressure without adding fuel. Increased cylinder pressure translates into more useable torque and power to maintain the power required for 'rolling resistance' of the vehicle down the road. The increase in horsepower often results in better fuel economy--although we do not guarantee mileage gains--as we cannot determine how YOU drive, and what your driving habits are.
Diesels are a different story...we need to stuff as much air in the cylinder as possible--and create maximum cylinder pressure at diesel injection point, use all the heat energy created to maximize pressure while keeping the total combustion event in the cylinder prior to exhaust valve opening. Smoke is wasted energy...in reality, diesels can actually run as low as 100lbs of air to 1 lb of fuel--although there's not enough heat energy created at that ratio. There's no magic air fuel ratio for diesel--leaner operation does not create damage, it just won't make power.
Therefore, you need a balance of fuel, best boost and proper timing to make the diesel efficient.
Lyndon Wester, Owner Westers Garage
